4. Conclusions
The following conclusions can be made based on the results.
When the target was at room temperature, a crater was formed and the radial flow of molten Cu outwards from the crater was observed, and many droplets were created.
When the thin target was heated (warm) (about 360 °C + 20 °C), a crater was developed, and capillary waves appeared in the vicinity around of the Cu crater. Approximately 30% smaller thermal gradient in the target decreased solidification time of the molten layer. The instabilities can be regarded as thermocapillary waves caused by the Marangoni effect. The melt will flow from the hotter regions to the colder ones. The wavelike surface formed by the previous pulses can increase the absorption of light in valleys, increasing their temperature and enhancing the amplitude of waves. The final target structures is similar to final states of chaos in Rayleigh- Bénard convection.
The main features of the surface topography and damage caused by laser irradiation originated from the non-homogeneous UV laser beam.
The current results give mainly qualitative information. It is necessary to continue with the experiments on heated targets with this type of laser, and also to make a better model for quantification of the results.